1. Field of the Invention
The present invention relates to external motion-guiding linkages for a human knee. More particularly, the present invention relates to an external motion-guiding load-bearing external linkage which can be positioned to accurately guide the motion of the knee while closely replicating normal motion and can be adjusted to either carry a required portion of the load or to distract and unload the knee completely.
2. Brief Description of the Prior Art
The knee joint is subject to many types of traumatic injuries and pathological conditions which result in soft tissue rupture, dislocation, bone fracture, cartilage erosion, or infection. The present methods of treatment are usually rest, bracing, casting, internal fixation, external fixation, ligamentous reconstruction, prosthetic replacement or a combination of the above.
Immobilization has been found to be less than satisfactory because it can reduce subsequent motion of the joint sometimes permanently. Soft tissue repairs are adversely affected by both immobilization and mobilization, unless the latter is prevented from overstretching the tissues before healing occurs. Internal fixation has the shortcoming that a long period of non-weight bearing is often necessary before adequate bone strength is achieved.
External linkages would not suffer from the above shortcomings but have been unsatisfactory in the past because they provide only approximate motion and are not compatible with the more exacting motion requirements of the internal structures of the knee. A system which accurately controls the motion of the knee and which allows a required amount of load-bearing would thus be of great clinical benefit and would enable further advances in surgical treatment.
However, accurately replicating the internal motions of the knee has been difficult because the motion of the knee cannot be represented as a simple fixed-axis hinge. Rather, it has been known that the joint surfaces of the knee undergo a combination of rolling and sliding, that the medial and lateral sides move differently and that there is a transverse rotation of the femur about the longitudinal axis of the tibia, especially towards the extension position. This is known as the "screw-home" mechanism.
The above can best be understood with reference to FIG. 1 which is a schematic representation of the joint as seen from the medial or lateral side. As seen there, the tibia 2 includes upper joint load-bearing surfaces 4 which are slightly curved and slope downward in a posterior direction by about 10.degree. relative to the longitudinal axis 6 of the tibia. The femur includes lateral and medial condyles whose load-bearing surface outlines can be approximated by circular arcs. That is, the centers of curvature of those portions of the load-bearing surface 10 which are in contact with the tibia at various angles of flexure are not identical. As seen in FIG. 1, at small degrees of flexure, the arc BC which is defined by the load-bearing surface has a center of curvature A. However, at greater flexure of the knee (dashed line position) the arc BD defined by the load-bearing surface has a center of curvature P which is different from A. Thus, a simple hinge having a single center of rotation would not accurately represent the complex motion of the knee joint.
Attempts have been made in the prior art to describe this knee flexion, extension motion by charting the path of the instant centers of rotation of the surface 10. In some cases, smooth curves were found while in others erratic curves were found. In any case, all of the plotted movements of the centers of rotation differed from knee to knee and from the medial to the lateral sides of a given knee. It has also been discovered that the instantaneous center of rotation of the surface 10 could be accurately represented by the rolling of fixed and moving centrode paths of simple geometry.
This concept of rolling one surface on another was utilized in the structure disclosed in "Restoration of Function in the Knee and Elbow with a Hinge Distractor Apparatus" by Volkov et al (see The Journal Of Bone and Joint Surgery, Vol. 57-A, No. 5, July 1975, pp. 591 to 600). In the design there described, two transverse pins through the femur and tibia held an adjustable rack and pinion on the medial and lateral sides as well as a locking mechanism anteriorly. Traction bows encircled the anterior halves of the shank and thigh. The rack and pinion arrangement utilized the concept of rolling one surface on another. However, this design was complex and did not accurately approximate the motion of the knee. The rack and pinion resulted in backward motion of the femoral condyles on the tibial condyles during flexion. Moreover, there were no differences in motion between the medial and lateral sides of the joint. A further problem was that varus and valgus were controlled statically by anterior locking bars, but not dynamically.
Orthotic devices generally used today employ a simple fixed-axis metal or plastic hinge set as close to the average center of rotation as possible, or a "polycentric" hinge similarly placed. These joints, as with Volkov et al, do not account for medial/lateral differences and for transverse rotation. The transverse rotation is taken up by the looseness in the device and by the soft tissues between the clamped device and the bone. None of the orthotic devices in use provide for more than approximate methods of placement. However, such is not possible when using transcutaneous pins which are fixed to the bone itself, and an external frame mounted on transcutaneous pins must allow for very accurate approximations of the knee motion.